Loose assemblies of staple fibers commonly referred to as "batts" must be bonded or secured in some fashion to make them into useful, easily handled and saleable nonwoven products. This requirement has lead to the development of not only various felting processes referred to as mechanical entanglement, but also to a great many types of chemical binders using either solvents or synthetic polymer dispersions. Additionally several processes have been employed wherein the energy of high pressure water jets is used to entangle the fibrous substrate. The latter processes are referred to as hydroentanglement or water-jet entanglement.
Mechanical entanglement processes bind or secure the fibers in the substrate by impaling the batts with a large number of barbed needles in a device called a needle loom. This action pushes fibers from the material's surface into the bulk of the batt. While strength properties are improved by this entangling of fibers within the batt, the process is slow, the needles damage the fibers and are themselves worn out rapidly, and the process is inherently suited only to the entanglement of heavy weight substrates.
The use of chemical binders also improves coherency and strength but has its own list of disadvantages. The substrate must be dried, dipped in the latex bonding solution, dried again, and heated to crosslink the polymer, thus markedly increasing the energy required to produce a final article. The polymeric latices also stiffen the final product, leading to the use of expensive post-treatments to soften the bonded web.
In order to avoid these problems nonwoven processes have been developed which use the energy of small-diameter, highly coherent jets of high pressure water to mimic the entangling action of the older needle loom. Initially, the water jet treating process involved the use of preformed dry-laid, fibrous web materials that were supported on an apertured surface so that the streams of water directed at the web material would move or separate the fibers and cause a pattern of varying densities and even apertures therein. In most instances, the resultant web simply evidenced a rearrangement of the fibers in the preformed sheet material, with the rearranged fibers exhibiting very little, if any, actual fiber entanglement. The rearrangement resulted from the use of water at a pressure sufficient to move the fibers sideways, but insufficient to entangle them effectively. Typical examples of this type of sheet material may be found in Kalwaites U.S. Pat. No. 2,862,251. These fiber-rearranged and apertured web materials frequently required significant amounts of binder to impart strength sufficient to permit further handling of the sheet materials.
It has also been found that high pressure water jets can be used as an entangling force operating on preformed nonwoven web materials prepared by carding or air laying. The jets of water entangle the fibers so that the material is held together by interfiber frictional forces in a way similar to that in which staple fibers are spun into a composite yarn for the production of conventional textiles. The patent to Guerin, U.S. Pat. No. 3,214,819, describes a method in which water jets are used to provide an entangling action similar to that provided with the barbed needles of a mechanical needle loom. However, this technique is perhaps best exemplified by the Evans U.S. Pat. No. 3,485,706. The technology further developed so as to provide entangled but non-apertured nonwoven material by using high pressure liquid jets and a relatively smooth supporting member as described by Bunting, et al in U.S. Pat. Nos. 3,493,462, 3,508,308, and 3,620,903.
The resultant entangled materials exhibited advantageously improved physical strength and softness relative to either mechanically entangled materials, or those fabrics which were bonded by chemical binders. The binder-free fabrics are not stiffened by the polymeric material, the water jets do not damage the fibers as they entangle them, and the product can be patterned as part of its production process. For these and other reasons the hydroentanglement process has supplanted earlier processes for demanding end uses. However, there are inherent disadvantages in even this process. The energy required to produce strong binder free product is very large, and the equipment needed to provide very high pressure water jets is very expensive. A highly uniform starting web or batt is needed or the high pressure water will produce holes and other irregularities in the product. The width of product was limited by the width of machinery available to produce uniform starting material. More economical fluid entanglement processes which operate at somewhat lower water pressures have also been disclosed by Suzuki, et al in U.S. Pat. Nos. 4,665,597, 4,805,275 and by Brooks et al in U.S. Pat. No. 4,623,575.
In substantially all of these prior art techniques, a precursor or preformed web material was formed, generally by air laying or carding, and subsequently was subjected to entanglement by the water jet method. Although most precursor webs were formed by an air laying system or by carding, some preformed wet-laid web materials or papers have also been mentioned. The air-laid webs, however, have been preferred since they are believed best for providing the desired isotropic properties, that is, equal physical properties in both the machine and cross-machine directions. Where carding techniques were employed, a preformed web was typically made using a cross-laying technique to provide the appropriate fiber orientation.
When it is desired to incorporate wood pulp fibers into the final sheet material, techniques such as those disclosed in Kirayoglu's U.S. Pat. No. 4,442,161 and Shambelan's Canadian Patent 841,938 have been employed. As described in the U.S. patent, a very light preformed tissue paper is layered on top of a preformed textile fiber web and high pressure water jets are directed against the tissue paper to join the two in a process reminiscent of needle punching, by destroying the tissue's structure and forcing the wood pulp fibers into the textile fiber web to provide the desired integrated composite structure having improved liquid barrier properties. However, no claims are made for any enhancements in web strength as a result of the inclusion of the wood pulp fibers into the composite structure. The Canadian patent teaches entanglement of papermaking fibers containing up to 25% textile staple fibers, the entanglement taking place prior to the drying of the wet-laid sheet and without the use of adhesives. The patent emphasizes hydroentangling lamination of multiple layers.